Solar energy collectors of one conventional type are provided with solar energy absorbing panels disposed to be impinged by solar radiation and to absorb heat therefrom. Such heat is transferred to a fluid such as water which is circulated through piping located in thermally conductive relation to the panels. The fluid is conveniently forced through the piping circuit by an electrically controlled pump and thus the heat from the panels is absorbed by the fluid and eventually stored within a suitable storage tank into which the heated fluid is conveniently directed.
It is undesirable, of course, that the pump be used to pump fluid through the system when the panels are not heated such as, for example, at night or in daytime when solar radiation is unable to reach the panels in sufficient quantity to adequately raise their temperature. Accordingly, thermostats have been employed to detect the temperature of the collector and to turn the pump on and off in response to the collector temperature. Thus the pump can be made to operate only when heat is available from the collector. This is a function of the incident solar energy and the outdoor ambient temperature, and whenever these two parameters are such that the collector is substantially above the temperature of thermal storage, then useful heat is available.
In known instances where the temperature of the collector's absorbing panel is used as the criteria for determining whether a pump should be operated or shut off, it was found that many times the pump would be operated unnecessarily because of the close differential which is provided. A very small amount of solar input will increase the temperature of a panel by 50.degree. Fahrenheit or more, for example. However, the amount of heat that ia available in a collector whose equilibrium temperature is only a few degrees above storage is not enough to warrant running the collector pump.
For turning on a collector pump in the morning, for example, a simple thermostat measuring the collector temperature itself would be adequate, and a setting of up to 50.degree. over storage temperature would give a good compromise between picking up all available heat and running the collector pump more than necessary. However, once cold water is ciruclating in the collector, the collector temperature will drop and cause the pump to cycle off, since collector temperature was used for the measurement and switching point.